Current Organic Chemistry - Volume 10, Issue 18, 2006

After the success of the 1st Organophosphorus (OP) special issue of Current Organic Chemistry consisting of two parts published back to back it is my pleasure to announce the 2nd. OP special issue approved by the Editor-in-Chief, Professor Attaur- Rahman and the management of Bentham. The first three papers cover relatively large molecules, such as phosphorus-containing cyclodextrins, calixarenes and cyclophanes, as well as dendrimers. The phosphorus-containing macrocycles form an important group due to supramolecular host-guest interactions allowing enantioselective syntheses based on chiral discrimination and making possible selective extractions and application as sensors. Phosphorus containing dendrimers represent a marvellous field involving the features of art and a real architecture. The next article is on the fragmentation and rearrangements of α-hydroxyiminoalkylphosphonates. The generation of metaphosphates is important from synthetic point of view, as the low-coordinate P-species are efficient phosphorylating agents; nevertheless the mechanism for the fragmentation is also of interest. The rich chemistry of N-vinylic phosphazenes is reflected in the next contribution, as they can be excellent building blocks in the synthesis of a variety of acyclic and heterocyclic compounds. The issue of stereogenic phosphorus remains an evergreen topic from the point of view of chirality, especially if it is discussed in a broader context including tricoordinated-, tetracoordinated-, pentacoordinated- and hexacoordinated phosphorus. Two papers cover the chemistry of sterically congested phosphorus compounds, such as tert-butyl substituted phosphinic and phosphinous acid derivatives, as well as crowded triarylphosphines. The presence of bulky P-substituents may be important from the point of view stability and mechanism, but may bring about advantages connected with the physical properties of the substrate. The next three articles fall in the scope of P-heterocyclic chemistry, but from entirely different approaches. Recent developments of the family of azaphospholes, that can be regarded to be modifications of the most common P-heterocycles, phospholes, are discussed. Then an up-to-date method for the synthesis of P-ligands applying zirconium reagents is shown and this is followed by the presentation of chiral heterocyclic phosphines together with asymmetric catalytic applications. Finally, the phosphorus aspects of green chemistry have been surveyed. The use of quaternary phosphonium salts and 1,3- dialkylimidazolium hexafluorophosphates are summarized including the major synthetic applications. The 1. Organophosphorus Special Issue has been divided into two parts published back to back. The six reviews included in this issue (Part I) is to be followed by six other articles in Volume 11, Number 1 (Part II). The forthcoming papers are the following: The Chemistry of Phosphinic and Phosphinous Acid Derivatives Containing t-Butyl Group as a Single Bulky Substituent: Synthetic, Mechanistic and Stereochemical Aspects by Józef Drabowicz, Wies awa Kudelska, Andrzej opusi ski and Adrian Zaj c Synthesis, Structure, and Properties of Crowded Triarylphosphines by Shigeru Sasaki and Masaaki Yoshifuji Recent Advances in the Chemistry of Anellated Azaphospholes by Raj K. Bansal, Neelima Gupta and Surendra K. Kumawat Synthesis of Phosphorus Ligands from Zirconium Reagents. A Useful Approach by Maria Zablocka and Jean Pierre Majoral Synthesis of Chiral Heterocyclic Phosphines for Application in Asymmetric Catalysis by Jens Holz, Mandy-Nicole Gensow, O. Zayas and Armin Börner The Phosphorus Aspects of Green Chemistry: the Use of Quaternary Phosphonium Salts and 1,3-Dialkylimidazolium Hexafluorophosphates in Organic Synthesis by György Keglevich, Zoltán Baán, István Hermecz, Tibor Novák and Irina L. Odinets

Although still a relatively young field of research, supramolecular chemistry ranks amongst the most important disciplines of organic chemistry with massive number of papers published annually. Despite of their importance in biology and industry organophosphorus compounds appear quite rarely as an object of supramolecular chemistry studies. In this respect compounds able to form supramolecules with certain guest organophosphates and phosphonates have been studied to some extent whereas application of phosphorus containing host molecules is still in its infancy. In this review a state-of-the-art research showing current progress in design and application of host molecules able to recognize and bind specifically phosphate, phosphonate or phosphinate fragments is described. These molecules are usually designed as chemical models of chosen enzymes or for various analytical purporses (as components of sensors, extractants, chiral discriminators, membrane carriers ect.).

Macrocyclic compounds are ubiquitous as synthetic receptors in supramolecular chemistry. This paper provides an overview of studies in the synthesis and functional properties of phosphorus-containing chiral cyclodextrins, calixarenes and cyclophanes having three-dimensional molecular cavities, within the context of molecular recognition and catalysis. The review demonstrates that chiral phosphorus-containing macrocycles are fundamentally and technologically interesting, because they combine chirality and supramolecular chemistry. Incorporation of phosphorus-containing receptor groups on the macrocyclic cavity enable supramolecular interactions to be defined a priori, having broad ramifications for chemistry, physics, biology and material science.

The methods of synthesis of dendrimers and dendritic macromolecules possessing phosphorus derivatives at each branching points are reviewed. Classical methods using the repetition of two steps are first described, then various ways usable to shorten the time needed for the synthesis are emphasized. These improvements include the use of accelerated methods (one step for one generation), and of dendrons (dendritic wedges) to increase rapidly the number of generations. Beside classical dendrimers constituted by only one type of branches and one type of functional end groups, more sophisticated dendritic structures are described. In particular, up to 4 types of functional end groups can be grafted to each divergent point of the surface, or two types of functional groups can be located at the end of particular branches of special dendritic species. In the latter case, the dendritic species are obtained either by reacting functional groups included at particular layers of a main dendrimer, or taking advantage of the reactivity of the function located at the core of dendrons.

This article reviews the unique reactivities of α-hydroxyiminoalkylphosphonates, which result from the proximity and the interaction between the hydroxyimino and the phosphonic groups. The reaction course of α-hydroxyiminoalkylphosphonates depends markedly on the type of groups linked to the phosphorus and on the stereochemistry of the oxime function. (Z)- α-Hydroxyiminoalkylphosphonates and phosphinates undergo, upon heating, fragmentation to a nitrile and a phosphate or phosphonate, while (E)- α-hydroxyiminoalkylphosphonates and -phosphinates undergo Beckmann rearrangement to N-acylphosphoramidates or N-acylphosphonamidates, respectively. In contrast to the diesters, α-hydroxyiminoalkylphosphonic acids (and monoesters) are unstable and fragment to metaphosphates, which can perform in situ phosphorylation of appropriate groups. Kinetic and mechanistic studies of the fragmentations of -α hydroxyiminobenzylphosphonates to metaphosphoric acid and esters are reviewed. The fragmentation of α-hydroxyiminobenzylphosphonic acid monoesters to metaphosphate esters requires acid catalysis and slows down at about pH 3. In contrast, α-hydroxyiminobenzylphosphonic acid undergoes fragmentation in aqueous solutions at a wide range of pH, from strongly acidic up to about pH 9. The α-hydroxyiminophosphonate fragmentation methodology was applied to phosphorylation of silica gel, which can potentially be useful as chromatographic stationary phase. A variety of stable precursors of α-hydroxyiminophosphonates that can be induced to fragment thermally, photochemically or by base catalysis is described. (E)-α-Hydroxyiminobenzylphosphonamidates of various types have been found to undergo Beckmann rearrangement to yield N-acylphosphordiamidates upon heating in nonpolar solvents such as toluene, or fragmentation to metaphosphonamidate by heating in polar solvents. The fragmentation of (E)-α-hydroxyiminobenzylphosphonates to metaphosphates is considered a special case of the Beckmann fragmentation.

The synthesis and reactivity of the N-vinylic phosphazene derivatives is the subject of this review. The first part gives a survey of different methodologies for the preparation of N-vinylic phosphazenes. These N-vinylic phosphazenes can be prepared either, by Staudinger reaction of N-vinylic azides with phosphines, or by reaction of phosphorus ylides and nitriles. The second part covers the high synthetic potential of these N-vinylic phosphazenes as valuable building blocks in preparative organic synthesis. N-Vinylic phosphazenes can be used as starting materials for the preparation of enamines by hydrolysis under basic or acidic conditions, and can be transformed into aminophosphonium salts by treatment with hydrogen halides or acyl halides. However, the most important synthetic application of N-vinylic phosphazenes is their use for the construction of carbon-nitrogen double bonds by means of aza-Wittig reaction with carbonyl compounds. This aza-Wittig reaction of N-vinylic phosphazenes has been widely applied for the preparation of simple and functionalized 2-aza-1,3-butadienes. These 2-azadienes are excellent starting materials for the construction of acyclic and heterocyclic compounds, and more elaborated polycyclic derivatives. Also the reactivity of N-vinylic phosphazenes with cumulene double bonds as carbon dioxide, carbon disulfide, isocyanates, isothiocyanates, ketenes and triple bonds, and the use of this process for the synthesis of acyclic and cyclic derivatives has been reviewed.

This review is concerned with stereogenic phosphorus sites as they occur in phosphorus compounds owning the several stages of coordination. The nature of the stereogenic phosphorus, the modes of generation of compounds containing such stereogenic phosphorus, and the stereochemistry associated with their reactions are surveyed. In particular, tricoordinated (phosphine, phosphite, phosphonite and phosphinite) phosphorus, tetracoordinated (phosphonium, phosphonate, phosphinate, phosphine oxide) phosphorus, pentacoordinated (phosphoranes and oxyphosphoranes) phosphorus, and hexacoordinated (cationic and anionic) phosphorus systems are reviewed. In addition to efforts recently reported concerning such compounds, historical perspectives and potential topics for continuing research are presented. Experimental procedures for the preparation of representative chiral species of each type are given.